THE STRUCTURE OP CELLS 27 



recognition of the fact that the reduction may occur at morpho- 

 logically diverse stages in the life-history of the various organisms 

 a fact which clearly emphasises its profound physiological sig- 

 nificance. But although there is no lack of hypotheses to explain 

 it, no one as yet has given a satisfactory theory which will 

 embrace the whole range of the phenomena concerned. 



In the higher organisms the process of reduction appears 

 invariably to be closely bound up with two nuclear divisions which 

 rapidly succeed each other, and are hence often spoken of as the 

 Reduction Divisions. These differ in some important respects from 

 those characteristic of other mitoses. They can only be con- 

 sidered in outline here, and after premising the existence of a not 

 inconsiderable diversity as to the details of the process in different 

 organisms. In animals the mitoses in question only occur in direct 

 relation to the formation of the sexual cells or gametes, but in 

 plants it is more usual to find a greater or less number of cell- 

 generations follow on the Reduction Divisions before the actual 

 gametes are formed. Thus it becomes obvious that the formation 

 of sexual cells is not a necessarily immediate consequence of the 

 change in the nucleus. 



If the course of events be studied in an animal, it is seen that 

 in the development of the spermatozoon and of the mature egg, a 

 strictly comparable series of changes is passed through. Just as 

 the spermatocyte gives rise, by two successive bipartitions, to four 

 sperms, so the immature ovum, by means of two successive nuclear 

 divisions, gives rise to four potentially fertilisable eggs, of which, 

 however, three commonly degenerate and are known as the polar 

 bodies. 



The nucleus of the spermatocyte, just as does that of the im- 

 mature egg (which may be distinguished from the ripe egg by 

 the name of oocyte), goes through a somewhat prolonged period of 

 growth before entering upon the critical mitoses, As these two 

 divisions are marked by certain peculiarities from those of the 

 other cell-generations, it is convenient to designate them by special 

 names. 



The first may be termed the Heterotype, the second the 

 Homotype, mitosis, following the terminology introduced by 

 Flemming. During a large part of the growth -period, leading 

 directly to the heterotype division, the nucleus cannot be correctly 

 described as resting, for the linin reticulum is plainly discernible, as 

 also are the regularly arranged chromatin granules, which serve to 

 render it distinct. In fact, this prolonged spireme is highly 

 characteristic of the heterotype mitosis, as contrasted with those 

 which have been previously gone through. It is during this stage 

 that the fission of the chromatin granules occurs, as was first seen 

 by Pfitzner in 1881. Each granule becomes drawn out into a 



